A semiconductor wafer of, for example, silicon or gallium arsenide is produced in the form of a large diameter. This wafer is cut and separated (diced) into small element chips (IC chips) and is transferred to the subsequent mounting step. In this process, in particular, the semiconductor wafer in the state of being stuck to a pressure sensitive adhesive tape is subjected to dicing, cleaning, drying, expanding and pickup steps, and is transferred to the subsequent bonding step.
It is desired that the above pressure sensitive adhesive tape employed from the semiconductor wafer dicing step through the pickup step have an adhesive strength which is so large as to securely hold the wafer chips in the dicing to drying steps but which is on such a level that no adhesive adheres to the wafer chips in the pickup step.
The chips having been picked up are bonded to lead frames with the use of a die bonding adhesive, such as an epoxy adhesive, in the die bonding step. Thus, semiconductor devices are produced. However, when IC chips are extremely small, application of an appropriate amount of adhesive is difficult, and there has occurred such a problem that the adhesive overflows IC chips. On the other hand, when IC chips are large, there has occurred such a problem that the bonding with satisfactory strength cannot be attained because of, for example, an insufficient amount of adhesive. Further, the application of such a die bonding adhesive is laborious. Therefore, there is a demand for an improvement enabling simplification of the process.
For solving the above problems, various pressure sensitive adhesive tapes for wafer sticking which can perform both a wafer fixing function and a die bonding function have been proposed (see, for example, Japanese Patent Laid-open Publication No. 2 (1990)-32181).
Japanese Patent Laid-open Publication No. 2 (1990)-32181 discloses a pressure sensitive adhesive tape comprising a base and, superimposed thereon, an adhesive layer constituted of a composition which is composed of a (meth) acrylic acid ester copolymer, a general-purpose epoxy resin, a general-purpose photopolymerizable low-molecular compound, a thermally active latent epoxy resin curing agent and a photopolymerization initiator. This adhesive layer performs a wafer fixing function at the time of wafer dicing. After the completion of dicing, when exposed to energy radiation, the adhesive layer is cured with the result that the adhesive strength between the adhesive layer and the base is lowered. Thus, upon picking up of IC chips, the pressure sensitive adhesive layer is detached together with the IC chips from the base. The IC chips with the pressure sensitive adhesive layer are mounted on lead frames and heated. Consequently, the epoxy resin contained in the adhesive layer exerts a bonding strength to thereby finalize bonding of the IC chips to lead frames.
The adhesive tape for wafer sticking disclosed in the above publication enables so-called direct die bonding and enables omitting the step of applying a die bonding adhesive. That is, in the adhesive layer of the adhesive tape, all the components are cured upon the die bonding carried out through the energy radiation curing and thermal curing, so that the chips and lead frames are bonded to each other with extremely large strength.
Thereafter, generally, wire bonding is conducted via the reflow step.
For use in the reflow step, in recent years, solders not containing lead have been developed in order to cope with environmental problems. The melting temperature of the solders not containing lead is higher than that of conventional solder containing lead, thereby rendering high reflow temperatures inevitable. However, when the reflow temperature is high, even a small amount of water contained in the adhesive layer would vaporize and inflate, thereby causing the danger of package cracking.
Moreover, at the time of die bonding, it is demanded that the adhesive layer properly follow the contour of adherend surface. In particular, for enhancing the follow property at the time of thermocompression bonding, it is demanded that the elastic modulus of the adhesive layer be low at the time of thermocompression bonding at high temperatures. However, the above conventional adhesives are unsatisfactory in this respect. Therefore, there is still a demand for improvement on pressure sensitive adhesives.
The present invention has been made in view of the above state of the prior art. It is an object of the present invention to provide an adhesive tape having an adhesive layer which enables reducing the water absorption of adhesive curing product and which enables lowering the elastic modulus thereof at thermocompression bonding.